This invention relates to a ceramic multilayer circuit board and a process for manufacturing said board.
Recently, electronic circuit products have highly frequently been given high performance, miniaturized and made cheaper by using a ceramic circuit board. Processes for manufacturing the ceramic boards can be roughly classified into two types. One of the types is a green sheet process and the other is a thick film process. In the former process, a high melting conductor paste containing a metal, such as tungsten, molybdenum or molybdenum-manganese, is printed through a screen mask onto an as-formed alumina green sheet to form a circuit pattern and then the printed sheet is sintered at about 1600.degree. C. in a reducing atmosphere. In the latter process, the conductor paste is printed onto a sintered alumina substrate, which is then fired, and a resistor paste is printed onto the substrate, which is then fired. The conductor paste comprises a mixture of a conductive powder, such as a powder of silver, platinum, palladium or gold and a binder glass powder, which mixture is intimately kneaded with an organic vehicle. The resistor paste has approximately the same composition as that of the conductor paste, except that silver, palladium and ruthenium oxide are used as a conductive material. A high range resistance of a desired level can be obtained by controlling a ratio of the conductive material and the glass binder.
The green sheet process can easily produce a fine wiring (e.g. about 100 microns) printed and multilayered board, and thus it is suitable for obtaining a high density circuit board but it is not suitable for obtaining a needed resistance, because a resistor paste using a high melting metal cannot provide a high range resistance when sintered at a high temperature, e.g. 1600.degree. C., and such a resistance cannot be controlled even by trimming because of the hardness of the resulting resistor. On the other hand, the thick film process can easily produce a resistor because a resistor paste is fired at a temperature below 1000.degree. C. In other words, the thick film process can easily provide a precise resistance of a desired level by using various paste compositions or properly trimming. However, the thick film process is poorer in obtaining a high density wiring or stacking more layers than the green sheet process.
Combining the characteristics of the green sheet process with those of the thick film process has been considered in which a thick film resistor is formed by the thick film process on a ceramic substrate formed by the green sheet process and sintered, to make a high density circuit board.
Japanese Patent Laid-Open No. 14975/74 discloses a process for manufacturing an integrated circuit board comprising the steps of providing a ceramic green sheet having a heat resistant conductive layer applied thereon in such a manner that part of the layer to which an element is connected is exposed, sintering said green sheet in a reducing atmosphere, forming an oxidation resistant conductor as a protective layer on the exposed part either by plating or the thick film process and then connecting the element to said part by heat treating in an oxidizing atmosphere.
However, there is a critical problem in making a ceramic multilayer circuit board by the prior process that when a thick film resistor paste is fired in air a wiring conductor on a ceramic substrate is undesirably oxidized. As a wiring conductor metal to be applied on the ceramic substrate there is used a metal of so high a melting point that it is not molten at a temperature of sintering the ceramic substrate, such as tungsten, molybdenum or molybdenum-manganese. However, these metals are easily oxidized under conditions for firing a thick film resistor paste (temperature: 500.degree. to 1000.degree. C., term: 10 minutes and atmosphere: air). In order to prevent the wiring conductor from oxidization, a way of firing the thick film conductor and resistor in a neutral atmosphere such as nitrogen gas or a way of plating the conductor with a non-oxidizable metal are considered. In the former way, a conductor paste which is firable in a nitrogen atmosphere is already commercially available and obtains good results but, on the other hand, no good resistor paste which is firable in a nitrogen atmosphere and practicable has yet been developed. Thus, the former way is not yet practicable.
Therefore, such a process as shown in FIG. 1 has been adopted. In FIG. 1, a conductor wiring is formed by printing on an alumina green sheet, the multilayering is carried out and then the multilayered alumina green sheet is sintered, which is not different from the prior art. The feature of the process as shown in FIG. 1 is that a wiring conductor such as tungsten is plated with nickel after sintering the alumina green sheet, the nickel plating layer is plated with gold in order to prevent the nickel surface from being oxidized, and finally a thick film resistor paste is printed on the gold plating layer and fired. A cross-sectional view of the resutling multilayer circuit board is diagrammatically shown in FIG. 2. In FIG. 2, reference numbers 1, 2, 3, 4, 4A, 5, 5A and 6 represent an alumina insulative substrate, wiring conductor, multilayered wiring substrate, nickel plating layer (electrode for thick film resistor), nickel plating layer (soldering part), gold plating layer (electrode for thick film resistor), gold plating layer (soldering part), and thick film resistor, respectively.
In the structure as shown in FIG. 2, nickel plating layers 4 and 4A directly under thick film resistors 6 are reacted with gold plating layers 5 and 5A, so that nickel is diffused to reach the top surface of the gold plating layers. Therefore, the nickel diffused is contacted and reacted with the conductive components or glass components present in the thick film resistor paste to form a reaction product at the thick film resistor terminal. Thus, in the prior art, no stable resistance can be obtained. Furthermore, the prior art requires a gold plating layer of more than 4 microns in thickness in order to obtain a stable resistance and to prevent the nickel surface from being oxidized. Therefore, the ceramic multilayer circuit board thus produced has a drawback that the cost for manufacturing it is expensive.